Osteoclast differentiation is normally assumed to be achieved through the differentiation of cells via a monocyte/macrophage differentiation pathway. The Pax5 deficient mouse strain accumulates a progenitor cell population that has properties suggestive of a proB cell phenotype. Treatment with RANKL and M-CSF results in differentiation of osteoclasts from bone marrow as well as spleen. An examination of bone from Pax5 deficient mice has revealed severe osteopenic phenotypic. This Proposal will definitively characterize the progenitors in bone marrow and spleen that can differentiate into osteoclasts. The bone marrow will be fractionated into distinct cell populations based on expression of cell surface markers and each population will be assessed for its capacity to differentiate into osteoclasts. Attempts will be made to immortalize ell clones that retain the capacity to differentiate in response to RANKL and M-CSF. Such cell lines could be used to dissect the signaling pathways for osteoclast differentiation characterize mechanisms of cell fusion into osteoclasts and identify and study the roles of additional genes that contribute to the differentiation process.